Staged Decompression Diving
Beyond the No-Stop Limit
Recreational diving is defined, in part, by the no-decompression limit: the maximum time you can spend at a given depth before the dissolved nitrogen in your tissues requires planned stops on the ascent. Stay within the limit, do a 3-minute safety stop at 5 metres as good practice, and surface. Decompression diving begins exactly at the point where that limit no longer applies — where the dive has accumulated a nitrogen load that requires one or more mandatory stops at specific depths, and where surfacing without completing those stops produces a meaningful risk of decompression sickness.
Staged decompression diving is the practice of planning and executing those mandatory stops. It is the domain of technical diving and is governed by a different set of rules, training requirements and equipment standards than recreational open-water diving. Done properly, it opens access to depth ranges and bottom times that recreational limits exclude: the deep holds of Second World War wrecks, the base of walls that begin where recreational diving ends, and the extended bottom times that allow serious underwater photography or marine science at depth.
How Nitrogen Loading Works
The physical process behind decompression diving is Haldane's model of inert gas uptake and elimination, developed in the early twentieth century and refined continuously since. When a diver breathes compressed gas at depth, inert gas — primarily nitrogen in recreational and most technical diving, helium in trimix applications — dissolves into the body's tissues at a rate proportional to the pressure difference between the breathing gas and the tissue. Different tissues absorb and release gas at different rates; these are modelled as 'compartments' with distinct half-times.
During ascent, pressure decreases and the tissues release dissolved gas back into the bloodstream for elimination through the lungs. If ascent is too fast, the gas comes out of solution in the tissues before it can be eliminated, forming bubbles that cause decompression sickness. Decompression stops work by pausing ascent at a depth where the pressure difference drives gas elimination without allowing bubble formation — the stop allows fast compartments to off-gas while the diver waits for slower compartments to follow. The 3-metre stop at 5 metres in recreational diving is a shortened version of this; mandatory technical decompression stops at 21 metres, 18 metres, 15 metres, 12 metres, 9 metres and 6 metres are the full application.
Decompression Algorithms and Planning Software
The decompression algorithms used in modern technical diving include the Buhlmann ZHL-16 model, developed by Swiss physiologist Albert Buhlmann and published in comprehensive form in 1983, and its various derivatives with gradient factor modifications. The RGBM (Reduced Gradient Bubble Model) takes a different physical approach, modelling bubble nucleation rather than purely dissolved gas. Both are implemented in dive planning software and in dedicated technical dive computers.
Planning software — Deco Planner, V-Planner, Z-Planner, and the planning modules in computers such as the Shearwater Predator and Perdix — generates a decompression schedule for a planned dive: bottom time, maximum depth, gas choices, and ascent rate. The schedule specifies the time required at each stop depth, total ascent time, and the total decompression obligation. A dive that accumulates 20 minutes of bottom time at 50 metres on air might produce a 40-minute decompression obligation across multiple stops from 21 metres to 6 metres.
Gradient factors — applied as a percentage of the supersaturation limit at which the model schedules stops — are used to tune the conservatism of the schedule. Common choices are gradient factor 30/70 for conservative decompression, 45/85 for a moderate approach, and higher values for more aggressive (and faster) decompression. The optimal gradient factor choice is a subject of ongoing discussion in the technical diving community and depends on the diver's personal history, the depth and duration of the dive, and the breathing gases used.
Decompression Gases
The most significant variable in staged decompression is the breathing gas used during ascent. Air at depth produces a slow off-gassing rate because the ambient pressure is high and the partial pressure of nitrogen in the gas is relatively low. As the diver ascends and switches to enriched-air nitrox or oxygen, the partial pressure of nitrogen in the breathing gas drops dramatically relative to the dissolved nitrogen in the tissues, steepening the pressure gradient and accelerating elimination.
A common technical diving practice is to use a bottom gas (air or trimix) for the working depth phase, switch to 50% nitrox at 21 metres, and switch to 100% oxygen at 6 metres or 9 metres. The oxygen switch at 6 metres can reduce total decompression time by a significant fraction compared to completing the stops on air. This is why technical divers typically carry multiple cylinders: a bottom gas cylinder and one or two decompression gas cylinders, often called 'deco bottles' or 'stage cylinders', carried clipped to the harness.
Oxygen has its own risk. At partial pressures above 1.6 ata, oxygen becomes toxic to the central nervous system, producing convulsions without warning. The depth limit for breathing 100% oxygen is 6 metres (1.6 ata absolute pressure), a limit that is firm and non-negotiable. Nitrox mixes used at shallower parts of the decompression schedule are planned to stay within safe oxygen partial pressure limits at each depth where they will be breathed.
In-Water Procedure
A staged decompression ascent is more physically and mentally demanding than a recreational ascent. The diver must maintain precise depth control at each stop — a diver who ascends 2 metres above a stop depth while fidgeting may be accumulating decompression penalty, not eliminating it. Buoyancy control at the stops is therefore critical and must be nearly effortless, because cognitive attention is simultaneously occupied with timing each stop, managing gas switches, monitoring gas supply, and watching the computer or schedule.
Gas switches require procedure: signal the switch to your buddy, take one breath from the new regulator to confirm it is working and the gas is the expected one (wrong gas at depth is a real hazard), switch to breathing the decompression gas, and secure the bottom gas regulator. All deco bottles should be clearly labelled with the gas mixture and the minimum depth at which they can be breathed safely.
Current adds considerable complexity. A deco stop in even a mild current requires the divers to maintain depth and position, typically by deploying a deco line (a weighted line hanging from the dive boat or from a surface marker buoy) and holding onto it. In locations like the passes at Fakarava in French Polynesia or at the Blue Corner in Palau, where current is a constant feature of the diving, managing a decompression obligation in current is a skill that requires practice and preparation.
Open the map to find the deep wrecks, walls and blue-water sites where staged decompression becomes relevant — from the Thistlegorm in the Red Sea to the deep sections of Cocos Island.
Training Pathway
Technical decompression training begins at the recreational-to-technical boundary. PADI's TecRec pathway starts with Tec 40, covering dives to 40 metres with up to 10 minutes of decompression on air or nitrox. TDI's Decompression Procedures course and IANTD's Advanced Nitrox and Decompression Procedures combination are the standard technical entry points for other agencies.
Deep decompression diving using trimix — replacing some nitrogen with helium to reduce narcosis and decompression burden at depths beyond 50 metres — requires additional training at the advanced technical level. GUE's structured progression from Fundamentals through Cave 1 and 2 to Technical Diver emphasises team skills and standardisation of equipment and procedures to a degree that other agencies do not.
The prerequisite for any decompression training is solid rescue skills, reliable buoyancy and trim, comfortable management of the existing gas system, and — in practice — a level of composure underwater that technical instructors evaluate directly. Decompression diving is not inherently more dangerous than recreational diving, but the consequence of a procedure failure is more serious, and the learning curve for doing it competently requires both formal training and incremental experience.